Field of the Invention
A part of the fabrication process for semiconductor devices involves the formation of a field oxide to separate from each other the active regions in which the devices are to be fabricated. This part of the process generally involves the formation of a nitride layer mask over silicon with pad oxide thereon at what will later be the active region(s) with oxidation of the exposed (without nitride thereover) silicon to form the field oxide. A problem has been that some of the oxidizing species will migrate under the corner of the nitride layer and oxidize some of the silicon under the nitride layer, this phenomenon being known as "bird's beak" as well as by other names and generally not being controllable. In order to minimize the likelihood of such migration of an oxidizing species under the nitride layer to form additional silicon oxide with the underlying silicon, the prior art has removed a portion of the pad oxide at the corner formed by the silicon and nitride layer, regrown a thinner layer of oxide in the undercut region and then placed a sidewall nitride onto the nitride layer which also fills the remainder of the undercut region over the regrown oxide. The field oxide is then grown. There have been recent proposals with recessed isolation whereby the oxidant can be completely prevented from reaching the silicon surface under the nitride. For this case, it appears advantageous to provide for a small amount of oxidation at the corner to provide rounding for control of stress and field enhancement. Such "bird's beak" elimination is required as dimensions are scaled below 0.25 micron and become increasingly susceptible to stress and field emission leakages. In these approaches, it is difficult to balance the various steps to provide for optimum oxidation of the corner region while maintaining acceptable encroachment and an independently adjustable process is required to improve control of the process results.